Highly doped III-nitride semiconductors

ABSTRACT

A wide bandgap semiconductor material is heavily doped to a degenerate level. Impurity densities approaching 1% of the volume of the semiconductor crystal are obtained to greatly increase conductivity. In one embodiment, a layer of AlGaN is formed on a wafer by first removing contaminants from a MBE machine. Wafers are then outgassed in the machine at very low pressures. A nitride is then formed on the wafer and an AlN layer is grown. The highly doped GaAlN layer is then formed having electron densities beyond 1×10 20  cm −3  at Al mole fractions up to 65% are obtained.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/146,466, filed May 15, 2002, now U.S. Pat. No. 6,953,740 which iscontinuation-in-part of U.S. patent application Ser. No. 10/140,774,filed May 7, 2002, now U.S. Pat. No. 6,888,170 entitled HIGHLY DOPEDIII-NITRIDE SEMICONDUCTORS, which claims the benefit of priority to U.S.Provisional Patent Application Ser. No. 60/364,499, filed Mar. 15, 2002,the entirety of which is incorporated herein by reference.

GOVERNMENT FUNDING

The invention described herein was made with U.S. Government supportunder Grant Number 0123453 awarded by the National Science Foundation.The United States Government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to degenerate doping of high mole fractionNitrides, and in particular to increasing the conductivity of the AlGaNby high density doping for applications including light emitting diodes,lasers, transistors, light detectors, photovoltaic cells andthermocouple.

BACKGROUND OF THE INVENTION

UV emitters are formed using multiple quantum wells combined withelectrical contacts to the wells. Such contacts are formed of AlGaN insome UV emitters. The wells emit light when a voltage is applied acrossthe contacts. Difficulties in conventional approaches to UV emitterfabrication for short wavelengths are predominantly based on poorelectrical conductivity in high Al mole fraction AlGaN, and highresistance contacts to these materials.

The mole fraction of Al determines the bandgap energy (and wavelength ofemission) of AlGaN. High Al mole fractions produce a large bandgap—GaNis 3.4 eV (364 nm) and AlN is 6.2 eV (200 nm). The LED or laser requireshigher bandgap materials to surround the emitting materials to avoidself-absorption of light, and for the laser, to provide lower refractiveindex clad layers. Thus, to get shorter wavelength light, higher Al molefractions are required, and even higher Al mole fractions need to beelectrically conducting for the surrounding clad layers.

SUMMARY OF THE INVENTION

A method forms a highly doped wide bandgap semiconductor materials to adegenerate level. Impurity densities approaching 1% of the volume of thesemiconductor crystal are obtained to greatly increase conductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representation of the order of depositing layers to form ahighly doped layer of AlGaN.

FIG. 2 is a cross section representation of a highly doped n-typeAlGaN:Si formed on a substrate.

FIG. 3 is a cross section representation of a light emitting diodeformed with layers of highly doped AlGaN.

FIG. 4 is a cross section representation of a laser formed using highlydoped AlGaN.

FIG. 5 is a cross section representation of a transistor formed usinghighly doped AlGaN.

FIG. 6 is a cross section representation of a light emitting diodeformed with a highly doped p-type runnel junction.

FIG. 7 is a block representation of a molecular beam epitaxy machinetreated to minimize contamination of substrate work pieces.

FIG. 8 is a block diagram of use of the light emitting diode of FIG. 3as a UV source for a fluorescent light bulb.

FIG. 9 is a block diagram of use of the light emitting diode of FIG. 3as a purification element for a water filter.

FIG. 10 is a block diagram of use of the laser of FIG. 4 as a transducerelement in an optical digital storage device.

FIG. 11 is a block diagram of use of the laser of FIG. 4 as a transducerelement for an optical telecommunications system.

FIG. 12 is a block diagram of a biological detection system utilizinglight emitting diodes.

FIG. 13 is a block diagram of a further biological detection systemutilizing light emitting diodes.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, reference is made to the accompanyingdrawings, which form a part hereof, and in which is shown by way ofillustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and thatstructural, logical and electrical changes may be made without departingfrom the scope of the present invention. Ranges of values have beengiven for various parameters and expressed as typical, minimum andmaximum for various embodiments. These are approximate values and shouldnot be taken as absolutes. Further, potentially large variations inparameter values may occur for alternative embodiments without departingfrom the invention as claimed. The following description is, therefore,not to be taken in a limited sense, and the scope of the presentinvention is defined by the appended claims.

In FIG. 1 a two-inch diameter sapphire wafer 100, approximately 330microns thick is polished to yield the (1000) orientation. The backsideof the wafer 100 is lapped to a rough surface. Approximately one micronof Tungsten 105 is deposited on the back of the wafer in one embodimentfor improved efficiency in heating the wafer during growth. In oneembodiment, the wafer is formed of sapphire, but other alternativesinclude SiC, Si, bulk GaN, AlN or any other substitute.

FIG. 1 is an illustration of one process of forming a highly doped AlGaNlayer. While individual layers are described as being formed, suchlayers combine in further processing.

The wafer 100 is clipped into a wafer holder for loading into amolecular beam epitaxy (MBE) processing machine. The wafer is loadedinto a load lock and pumped to approximately 10⁻⁷ T. It is thentransferred to a transition chamber at 10⁻¹⁰ T where it is outgassed for1-hour holding time at approximately 400° C. typical, 200° C. minimum,600° C. maximum to remove atmosphere contamination from the wafer andwafer holder. The temperature is raised and lowered from roomtemperature to high temperature hold, and back again, over 30 minuteperiods to avoid wafer shattering due to thermal stresses.

Substrate temperatures below 490° C. are estimated from measurements ofa thermocouple located a few mm behind the back of the wafer, whichreceives radiant heat from the wafer. Higher temperatures are measuredwith an optical pyrometer aimed at the front surface of the wafer.Sapphire and the deposited nitride semiconductor materials are mostlytransparent to the measurement wavelengths used by the pyrometer. Thetemperature measurements are not precisely the same as the nitridelayer. The temperature measurement is most closely related to thetemperature of the tungsten, or other metal, coating on the back of thesapphire wafer. The pyrometer temperature measurement is used in thismanner as a process control tool.

The wafer is then loaded into a growth chamber at approximately 10⁻¹⁰ Ttypical, 10⁻¹¹ T minimum and 10⁻⁹ T maximum. The chamber has beenpreviously treated to remove moisture, oxygen and other possiblecontaminants as described below. The substrate temperature is thenraised to approximately 200° C. +/−100° C. after the wafer is brought tothe growth position, where it faces furnaces and an RF source, and allsource shutters remain closed. Nitrogen is then fed to the RF source ata flow rate of approximately 0.7 sccm typical, 0.5 sccm minimum, and 0.9sccm maximum. The source power is approximately 500 W typical, 250 Wminimum, 600 W maximum.

After the flow has stabilized (approximately 15 minutes typical, 5minutes minimum, 20 minutes maximum), the shutter of the RF source witha source power of approximately 500 W typical, 250 W minimum, 600 Wmaximum, is opened to nitridize the surface 110 of the sapphire wafer105. The shutter is closed and the wafer is heated to approximately1033° C. typical, 900° C. minimum, 1100° C. maximum, as measured byoptical pyrometer over 30 minutes. During this time, the RF power isbrought to 250 W typical, 200 W minimum, 350 W maximum and flow isreduced to 0.4 sccm typical, 0.2 minimum, 0.7 maximum.

An AlN layer 115 of 100 nm+/−50 nm is grown by opening the Al and RFshutter together for approximately 1 min 50 sec typical, 1-minuteminimum, and 3 minutes maximum. Al, Ga and Si temperatures are measuredby a thermocouple in contact with the outside of the pyrolytic boronnitride crucible which contains the furnace contents. The temperaturereadings are all higher than the actual temperature of the furnacecontents due to this physical design. Furnace temperature values areused as process control tools and do not measure actual source materialtemperatures. The Al temperature is approximately 1276° C. typical,1225° C. min, 1325° C. max).

The Al temperature is then lowered to approximately 1205° C.+/−50° C.while all shutters are closed for approximately 1 minute 10 sec. The RF,Al, Ga and Si shutters are then opened. The Ga temperature isapproximately 1167° C.+/−50° C. and Si temperature is adjusted fordoping concentration. A Si temperature of approximately 1425° C. givesan electron density of 1×10²⁰ cm−3 in AlGaN with Al compositions from 47to 65% in one embodiment. Al temperatures above 1225° C. give about 47%Al mole fraction, while Al temperature of 1225° C. gives about 65% Almole fraction. The shutters remain open for approximately 1 hr 10 minwhich results in 400 nm thick layer 120 of AlGaN:Si. The wafer is cooledover 15 minutes after growth.

FIG. 2 is a cross section representation of N-type AlGaN:Si at 210formed on a sapphire substrate 220 via the process of FIG. 1. Othersubstrate alternatives comprise SiC, Si, bulk GaN, AlN or othersubstitutes. The choice of substrate is not likely to influence highlyconducting AlGaN characteristics. Doping levels have been obtainedconsistent with n-GaN beyond 3×10²⁰ cm⁻³, and n-Al_(0.65)Ga_(0.35)N andAl_(0.65)Ga_(0.35)N beyond 1×10²⁰ cm⁻³ electron concentration. Thestructure in FIG. 2 serves as a test structure used for Hall and CVmeasurements.

Hall effect measurements determine electron density and mobility inAlGaN. These measurements are conducted at room temperature to evaluateelectrical resistivity for application to electronic and optoelectronicdevices. Temperature variable Hall measurements provide electron densityas a function of temperature from room temperature down to 10 Kelvin toevaluate impurity ionization energy. When electron densities beyond1×10²⁰ cm⁻³ are measured at room temperature, the density is beyond thedegeneracy minimum limit for the conduction band of AlGaN. Temperaturevariable Hall measurements confirm that there is no change in electrondensity with temperature, which indicates that doping concentrationbeyond the degeneracy limit has been achieved.

The invention avoids the problem of self-compensation of dopants where alimit in electrical activity is reached with increasing dopantconcentration. In self-compensation of n-type materials, donorimpurities will reach a limit of incorporation onto donor sites, andbegin to incorporate on acceptor, or other deep level sites.Self-compensation is avoided when most of the impurities are located atdonor sites in the crystal. This condition occurs most readily forcrystals that have high degrees of perfection, and low levels ofunwanted impurities. The invention of this technique to reach degeneratedoping concentrations in AlGaN is the result of reducing mechanisms forpremature onset of compensation of donor impurities, such as thosefrequently encountered in the growth of AlGaAs on GaAs substrates.

FIG. 3 is a cross section representation of a light emitting diodeformed with various layers of highly doped AlGaN. N-type AlGaN:Si isformed as a layer 310 on top of a substrate 320. Light emittingstructures 330, referred to as multiple quantum wells (MQW) are formedon top of the n-type AlGaN:Si. MQWs are well known as structures used inlight emitting or laser diodes consisting of stacks of p-type and n-typewide band gap materials surrounding the MQWs. N-type ohmic contacts 340are also formed on top of the n-type AlGaN:Si layer 310 outside of theMQWs.

A p-type AlGaN:Mg layer 350 is then formed on top of the MQW layer 330in a manner virtually identical to the formation of the n-type AlGaN:Silayer 310, with Si being replaced by Mg, Be, C or Li to form acceptors.The p-type AlGaN:Mg is also highly doped and therefore highlyconductive. In one embodiment, the low oxygen growth techniquesdeveloped for the n-type doping are applied to similarly raise thep-type doping efficiency.

An alternative to using a single acceptor species is to co-dope theacceptor atoms with donor atoms such as Si. In this embodiment, holedensities beyond the co-doped donor density are obtained. The high donordensity of the invention can be applied to establish a higher holedensity in p-type AlGaN. P-type ohmic contacts 360 are formed on top ofthe p-type AlGaN:Mg layer 350. In response to a voltage applied acrossthe ohmic contacts 340 and 360, light is emitted from the multiplequantum wells 330 in a direction consistent with an arrow 370. Arrow 370extends from the multiple quantum wells 330 through the p-type AlGaN:Mg,which is essentially transparent to such light.

The mole fraction of Al determines the bandgap energy (and wavelength ofemission) of AlGaN. High Al mole fractions produce a large bandgap—GaNis 3.4 eV (364 nm) and AlN is 6.2 eV (200 nm). The LED or laser requireshigher bandgap materials to surround the emitting materials to avoidself-absorption of light, and for the laser, to provide lower refractiveindex clad layers. Thus, to get shorter wavelength light, higher Al molefractions are required, and even higher Al mole fractions need to beelectrically conducting for the surrounding clad layers. The inventionallows shorter UV wavelength light than could be emitted through presentelectrically conducting compositions.

A laser embodiment using the highly doped AlGaN materials is shown inFIG. 4. N-type AlGaN:Si is formed as a layer 410 on top of a substrate420. Multiple quantum wells 430 are formed on top of the n-type AlGaN:Silayer 410. N-type ohmic contacts 440 are also formed on top of then-type AlGaN:Si layer 410 outside of the MQWs.

A p-type AlGaN:Mg layer 450 is then formed on top of the MQW layer 430in a manner virtually identical to the formation of the n-type AlGaN:Silayer 410, with Si being replaced by Mg to form acceptors. The p-typeAlGaN:Mg is also highly doped and therefore highly conductive. P-typeohmic contacts 460 are formed on top of the p-type AlGaN:Mg layer 450.In this embodiment, the p-type ohmic contacts 460 completely cover thep-type AlGaN:Mg layer. In response to a voltage applied across the ohmiccontacts 440 and 460, light is emitted from the multiple quantum wells430 in a direction perpendicular to a line drawn between the two highlydoped layers 450 and 410. Light emission is thus essentially coming outof the sheet containing FIG. 4. Light in the laser is confined by higherAl mole fraction cladding layers. The p-contact and conducting p-regionsneed to be very low resistance for the laser. For the LED, the p-typecontact is in the way of emitted light, thus it needs to be smallcompared to the light emitting area. In this case, the p-typeconductivity is also very important because the distance from thep-contact to the emitting region increases, and conductivity must takeplace through the poorly conducting p-type material (hole mobility ismuch lower than electron mobility, and resistivity is proportional to1/neu where n is density, e is electron charge and u is mobility. Thehighly doped AlGaN layers are also useful in forming transistors. Anexample of one such transistor is shown in FIG. 5 at 500. A GaN layer510 is formed on a substrate 520 such as a sapphire substrate. A twodimensional electron gas (2DEG) GaN layer is deposited via molecularbeam epitaxy from plasma 525. A layer of AlGaN 530 is then formed on thegas layer 525. A highly doped n-type AlGaN:Si layer 535 is then formedin a manner consistent with the process of FIG. 1, followed by anotherlayer of AlGaN 540 and a GaN layer 550. A Schottkey gate contact 560 isformed on top of the GaN layer 550, and a source ohmic contact 570 isformed adjacent a first side of the structure with drain ohmic contact580 formed opposite the structure. The ohmic contacts 570 and 580contact part of GaN layer 510, and all of the layers formed above theGaN layer 510 in one embodiment. The highly doped AlGaN permits highersheet density in the 2DEG, thus higher transistor channel current.

In FIG. 6, a structure providing a tunnel junction contact to p-typeAlGaN is shown at 600. Multiple high-doped layers are utilized instructure 600. A first n-type AlGaN:Si layer is formed at 610, and hasan MQW layer 620 and n-type ohmic contacts 630 formed thereon, somewhatadjacent to each other. A p-type AlGaN:Mg layer 640 is then formed ontop of the MQW layer 620, followed by a n-type AlGaN:Si layer. Finally,another n-type ohmic contact 660 is formed on top of the n-type AlGaN:Silayer 650. The tunnel junction replaces the p-type ohmic contact inlight emitters and LEDs. The highly conducting n-AlGaN provides a lowerresistivity alternative to p-type AlGaN. This permits smaller metalcontact layers that block less light from LEDs that emit through the topcontact layers. For lasers, the tunnel junction provides lower laserseries resistances, and more uniform current injection.

MBE Chamber

A MBE apparatus conditioned to minimize contaminants is illustrated at700 in FIG. 7. In one embodiment, it has three ultra-high vacuumchambers, two growth chambers 710 and 715 and an intermediate chamber720. Intermediate chamber 720 placed between the two growth chambers 710and 715 to connect them to each other. A wafer-introducing chamber iscoupled to the intermediate chamber 720. A wafer (i.e., a substrate), onwhich semiconductor lasers are to be fabricated, is introduced into theMBE apparatus via the wafer introducing chamber, and then transferredinside the MBE apparatus from one chamber to the other.

To obtain high vacuum levels in selected chambers, a turbo-molecularpump 730 (simply indicated as the “turbo pump” in the figure) is coupledto growth chamber 710. Further pumps are coupled to other chambers asdesired. The turbo pump 730 is capable of evacuating the growth chamber710 to an ultra-high degree of vacuum. A rotary pump 735 coupled to theturbo pump 730 is also utilized.

Ion pumps 745 and 760 are coupled to intermediate chamber 720 and growthchamber 715 respectively. A cryo-pump 750 is coupled to respectivechambers and is used to establish ultra high vacuum in the intermediatechambers. Furnaces also contain resistively heated pyrolytic boronnitride crucibles which contain Al, Ga and Si.

In one embodiment, a wafer (i.e., a substrate), on which semiconductorlight emitting diodes are to be fabricated, is introduced into the MBEapparatus 700 via the wafer introducing chamber 725, and thentransferred inside the MBE apparatus from one chamber to the other.Further chambers also have evacuation equipment for creating ultrahighvacuum conditions in selected chambers.

MBE Chamber maintenance is performed using standard, and customizedultra-high vacuum (UHV) techniques. When the MBE machine requiresrecharge of materials, or repair of broken items, it is brought toatmospheric pressure using nitrogen gas. All parts are handled withgloves to avoid contamination. The Al, Ga and In furnaces have cruciblesinstalled for outgassing. Old crucibles are etched in Aqua-Regia (HCland Nitric acid), rinsed in DI water and heated to dry. These recycledcrucibles are used for Ga and In. Only new crucibles are used in the Alfurnace because their use avoids a problem of Al creeping up the insideof the crucible and running out of the top, which ruins a furnace. Si isnot routinely replaced. The same source has been employed for more than10 years in one embodiment, however, it is anticipated that such a longperiod may not be required, and that other methods of reducingimpurities in an Si furnace may be used The long period of operation athigh temperatures contributes to very high purity operation of the Sifurnace. Other undesirable impurities which may have been part of theoriginal Si charge material have been reduced in concentration byprolonged high temperature furnace operation. Unwanted impurities are tobe avoided to prevent undesired compensation of Si donors in AlGaN.

These crucibles are slowly individually raised in temperature and bakedat 1500° C. for 1 hour. They are removed a day later and filled with Ga,In and Al. The machine is pumped down to UHV and checked for leaks. Themachine is then surrounded by heater panels to form an oven and it isbaked at temperatures near approximately 150° C. as measured by athermocouple inserted inside the bake panels for 3-4 days.

In one embodiment, the substrate heater is raised in temperature to1000° C. over 10 hours during the bake procedure. It is left at thistemperature for 2 days until the bake is finished. This step providesout gassing of the substrate heater to remove impurities prior toutilization for AlGaN growth following the bakeout. If this step is notperformed, there will be significant out gassing of impurities duringAlGaN growth from the substrate heater. The out gassing of impuritiesduring AlGaN growth will contaminate the AlGaN layer with oxygen, andpossibly carbon or other materials which are known to produce undesiredphysical properties in most semiconductor crystals, and are oftenresponsible for limiting electrical conductivity, and opticalrecombination efficiency. Secondary ion mass spectroscopy measurements(SIMS) of GaN grown in this machine show that oxygen levels areexceptionally low. These measurements show that this machine canreproducibly grow GaN with oxygen and carbon concentrations less thanthe detection limit of 5×10¹⁶ cm⁻³. These levels are lower than anypublished GaN data.

To insure that this data is reliable, in one example, a growth of GaN inthis machine was performed on top of GaN grown elsewhere. The SIMsmeasurement clearly shows that GaN material from this machine hasundetectable oxygen and carbon, while the other GaN underneath shows thehigher, measurable levels typical of the best reports in the literature.

Heating the substrate heater during the bakeout removes these impuritiesmore effectively than trying to accomplish the same end result when themachine is not being baked. Removing the impurities while the machinewalls are hot will aid in keeping the impurities from sticking to thewalls where they will then slowly outgas during growths. This procedureis probably not followed elsewhere because it carries a significant riskof heater failure for such a prolonged operation when all of theinterconnecting leads are also at elevated temperature. A poorelectrical connection which momentarily interrupts heater current underthese conditions will usually destroy the expensive heater.

In a further embodiment, a light emitting diode such as that shown inFIG. 3 is utilized as a UV emitter for a fluorescent light bulb showngenerally at 800. The light emitting diode is shown packaged in atransparent material for protection at 810. The transparent material isalso coated with a fluorescing material 845 on the inside, such asphosphor to provide visible light. The light emitting diode 810 issupported by leads 815 and 820 which are coupled to a power conversioncircuit 830 for converting high voltage supplies, such as householdcurrent into the low voltage utilized by the light emitting diode 810.The voltage is above the bandgap voltage, such as above 4 volts in oneembodiment. 10 volts should be sufficient. Power conversion circuit 830comprises a battery, such as a 9 volt battery commonly available in afurther embodiment.

A case, such as glass, or a polymer is shown at 840, providing supportfor the elements of the light bulb, and also providing furtherprotection is shown. It is filled with an inert gas in one embodiment toreduce oxidation of parts. The gas is not required, but if used, helpsprevent degradation of the fluorescing material 845. Gases such asnitrogen, argon, or even a vacuum may be used. In one embodiment, thegas is selected to also be non-absorbing with respect to UV light. Thecase 840 may also take on characteristics of common light bulbs such asclear versus frosted, and may be any shape desired for decorativepurposes.

In a further embodiment, a water purification system is shown at 900 inFIG. 9. A UV light emitting diode 910 such as that shown in FIG. 3 isutilized as a water purification element. It emits light at about 280 nmwavelengths to effectively kill biological materials such as bacterialin water flowing through a channel 920 between two reservoirs 930 and940. Not shown, but implicit are pumping mechanisms and inlets andoutlets in the reservoirs. Multiple channels with multiple lightemitting diodes are utilized in one embodiment to maximize throughput.

A laser such as that shown in FIG. 4 is used as a transducer element inan optical storage device 1000 in FIG. 10. A optical media 1010 isrotated by motor 1020 in one embodiment. A linear actuator 1030 is usedto radially move a laser 1040 formed with the transducer element tofollow tracks on the optical media 1010. Information is written onto andread from the tracks using the laser. A housing 1050 provides protectionand support for components of the storage device 1000.

In a further embodiment, a laser such as that shown in FIG. 4 is used asa transducer element in a telecommunications system 1100 as shown inFIG. 11. An electrical input signal 1110 is received by a transducer1115. The transducer 1115 contains circuitry 1120 that receives theinput signal and converts it to a signal compatible with driving thelaser 1130, which is optically coupled to a fiber optic line 1140. Inthis manner, electrical signals are converted to optical signals forfurther transmission.

In yet a further embodiment, a biological hazard detection system isshown in FIG. 12. The detection system has a light emitting diode 1210emitting light with a wavelength of approximately 280 nm that is used todetect biological hazards. The light emitting diode 1210 is positionedon one side of a support structure 1220, with a photodetector 1230 alsopositioned on the support structure 1220 or some other structure. Thelight emitting diode 1210 emits light toward the photodetector. Thesupport structure 1220 allows suspected biological materials to passbetween the light emitting diode 1210 and the photodetector 1230. In oneembodiment, the biological materials fluoresce in response to light ofthat wavelength. The fluorescence is detected by the photodetector 1230and is indicative of the presence of the biological material. Both theamplitude of the fluorescence, and the frequency of the fluorescence maybe utilized by appropriate circuitry to identify the biological hazard.

In an alternative biological hazard detecting system shown in FIG. 13, afluorescing material 1305 is placed proximate a light emitting diode1310 and one or more photodetectors 1320. Photodetectors 1320 optionallyhave a bandpass filter 1330 disposed between the photodetectors and thefluorescing material 1305. Biological materials 1340 accumulate on thefluorescing material 1305, causing it to fluoresce in the presence oflight from the light emitting diode 1310. The photodetector 1320operates as above to provide at least one of intensity and frequency tocircuitry to identify the presence or concentration of the biologicalmaterial, or to identify the material itself. The elements are supportedby a suitable support structure 1350 allowing biological materials toaccumulate on the fluorescing material 1305.

In further embodiments, lasers constructed in accordance with theinvention in combination with photodetectors are utilized to detectclouds of biologically hazardous materials from a distance. The locationof the photodetectors may be any place desired where fluorescing of thebiological materials can be observed. In one embodiment, both the laserand photodetector are on a plane or the ground or other ground vehicleor structure. In a further embodiment one is on a plane and the other onthe ground, or a different plane such as a drone.

CONCLUSION

A technique for obtaining high carrier densities in wide bandgapsemiconductors is described. Dopant impurities in wide bandgapsemiconductors (Eg>4 eV typical) exhibit large ionization energiescompared to those encountered in smaller bandgap semiconductors. Largeionization energies result in small fractions of impurities which areionized at operating temperatures near room temperature (approximately296° K). The ionization energy barrier is effectively removed, enablingionization of most of the dopants. This results from employing what istermed heavy doping density (HDD).

Electrical conductivity in semiconductors is increased by the additionof dopants to add electrical charge. Impurities which add (or donate)mobile electrons are called donors. Impurities which remove (or accept)electrons are called acceptors. Acceptors are said to create mobileholes which have positive charge that conduct electricity, similar tomobile electrons. These mobile charges require sufficient energy toremove them from their host impurity. This energy is called theionizaton, or activiation energy. The density of carriers which resultfrom ionization of impurities is:n=Nd exp (−Ea/kT)

-   where, n=density of electrons (cm−3)-   Nd=density of donor impurities (cm−3)-   Ea=activation energy (eV)-   K=Boltzman constant (eV/K)-   T=temperature (K)

Activation energy is related to effective mass and relative dielectricconstant, which are related to bandgap. Dopants in larger bandgapsemiconductors typically have larger activation energy as shown in thefollowing equation:Ea=13.6*(m*/m ₀)/(∈_(r)/∈₀)²)

In variable composition material such as AlGaN, activation energy fordonors is approximately 30 meV for GaN and 350 meV for AlN. As a resultof high activation energy, very few carriers result from impurityionization when compositions of AlGaN close to AlN are made. The densityof ionized impurities is normally too low to add significant electricalconductivity to this material.

These formulations for ionization energy no longer apply when largeimpurity densities (approaching 1% of the volume of the semiconductorcrystal) are utilized. A new behavior for ionization of dopants takesplace. This new behavior appears to take place when the dopant atomsoccupy the atomic sites required for their donor or acceptor behavior.In one embodiment, Si doping atoms are added as donors to AlGaN. Whenthese very large impurity densities, on the correct crystal latticesite, are successfully achieved, the orbits of valence electrons ofdonor atoms begin to overlap with one another. The ionization energiesof the donor atoms are altered by the presence of the other nearby donoratoms. The result is that the energy barriers fall away to negligiblevalues. Proof of the lack of significant energy barrier is thetemperature variable Hall effect measurement of electron density as afunction of temperature. No significant loss of carrier density isexperienced as temperature is lowered from 300K to 20K. This behavior isin contrast to the commonly utilized semiconductor doping densitieswhere valence orbits interact only with the host semiconductor, whichgives rise to the more typical thermal ionization barriers.

The ionization energy changes under heavy doping density (HDD) from anarrowly defined thermal ionization energy to a band of energy, where,for example, the donor band overlaps the conduction band in n-typematerial. This conduction occurs when the degeneracy of the band isexceeded—more carriers are available than required to fill the lowerenergy portion of the conduction band. The material is also described asbeing degenerate under controlled incorporation of heavy dopingdensities of impurities. In one embodiment described above, Si dopingAlGaN at compositions up to 65% Al mole fraction, with electrondensities up to 1×10²⁰ cm⁻³ is obtained. This behavior can be obtainedin other wide bandgap semiconductor systems, when sufficient control ofthe dopant atom placement within the crystal lattice is achieved. Recentresults using the above techniques for doping AlGaN provide electrondensities approaching 1×10²⁰ cm⁻³ in Al compositions of 78-79%,demonstrating that the process is applicable to other wide bandgapmaterials.

The method of overcoming ionization limits to carrier densities isapplicable throughout wide bandgap semiconductors where ionizationenergies severely limit conductivity. Some of such semiconductorsinclude:

-   -   AlN    -   BN    -   Diamond    -   Other diamond-like semiconductors such as carbon nitride, or        carbon-based nanotubes    -   Diamond-like nitrides    -   Other carbon-based semiconductor-like materials such as        fullerenes (bucky-balls).    -   Other doped glasses, oxides and ceramics

The techniques described for heavily doping AlGaN may be used on thesewide bandgap materials, and others. However, actual ranges of growthconditions will likely vary for different materials. In any event,determining such ranges will likely simply be a matter of trial anderror, starting with the ranges used for AlGaN.

Further potential applications extend beyond obtaining higher electricalconductivity for conducting current, to other physical properties whichdepend on mobile carriers and ionized impurities. Added electron densityalters index of refraction. The selective introduction of heavy dopingdensity with location could be used as the basis for guiding light inoptoelectronic components and systems through control of refractiveindex.

Overcoming thermal ionization energies with the invented technique couldalso be applied to quantum wells, quantum wires, quantum dots andquantum dots made from wide bandgap materials for application to lightemitters, transistors, light detectors and solar cells. Photovoltaiccells which convert photons, or atomic particles, at energies far abovevisible light will require electrical conductivity for extractingelectrons generated by photon or particle absorption. Suitable materialswill possess very large bandgaps (10's to 100's of eV) and will requireheavy doping density to overcome thermal ionization limits to electricalconductivity.

While the concept of heavy doping density to overcome thermal ionizationhas been demonstrated for donors, it applies equally to ionization ofacceptor impurities. The key element is achieving sufficient acceptordensities on electrically active lattice sites. Because hole effectivemass is larger than effective mass in most semiconductor structures, thedoping density required to overcome ionization limits is higher. Inpractice it becomes more difficult to dope wide bandgap materials p-typethan n-type (e.g. —GaN and AlGaN). The invented method of eliminatingthermal ionization barriers through heavy doping density provides atechnique to obtaining high conductivity p-type materials as well asn-type materials.

In one embodiment, a method of forming a highly doped layer of AlGaN, ispracticed by first removing contaminants from a MBE machine. Wafers arethen out gassed in the machine at very low pressures. A nitride is thenformed on the wafer and an AlN layer is grown by opening Al and RFshutters. The highly doped AlGaN layer is formed by opening RF, Al, Gaand dopant shutters. Degenerate doping concentrations are obtained whicheliminate barriers to impurity ionization.

Injection of holes into AlGaN through n⁺/p⁺ tunnel junctions is used toreplace thick p-AlGaN with n-AlGaN as a contact to multiple quantum well(MQW) emitters. High doping concentration provided by the currentinvention further reduces AlGaN resistance, and reduces the alreadysmall parasitic voltage drop and series resistance introduced by thetunnel junction. In one embodiment, n-GaN beyond 3×10²⁰ cm⁻³, andn-Al_(0.65)Ga_(0.) beyond 1×10²⁰ cm⁻³ electron concentrations areobtained. Electron densities in AlGaN with Al mole fractions up to 65%are more than 1×10²⁰ cm⁻³ where previous reports only showed doping inthe 10¹⁷-10¹⁸ cm⁻³ range at compositions of 45% and 30% respectively. Noprevious techniques have shown electrical conductivity usable for Alcompositions above 50%. The invented technique thus has improvedelectrical conductivity by 100-1000 times, or much more, depending onspecific Al mole fraction. This improvement reduces parasitic voltageloss to levels where high efficiency light emitting diodes, includinglasers, can be made.

These levels of doping at high Al mole fraction provide application ofn-type bulk, and n/p tunnel injection to short wavelength UV emitterswith very high wall-plug efficiency. Ohmic contact resistance to MQWs isgreatly reduced with such doping levels. Such doping levels provideohmic contacts to undoped Al_(0.65)Ga_(0.35)N.

Further embodiments include light emitting diodes having wavelengthsbetween approximately 254 and 290 nm for use in fluorescent light bulbs,photonic transmssion and detection, biological, explosive and toxicmaterial detection, water purification and other decontaminationenvironments. Lasers formed using the highly doped layers are useful inhigh-density storage applications or telecommunications applications. Inyet a further embodiment, a transistor is formed utilizing the highlydoped layer as a channel.

In a further embodiment, a molecular beam epitaxy (MBE) machine isspecially maintained and utilized to provide an environment wherenitrides are grown free of detectable levels of oxygen for highlyefficient doping of materials, such as AlGaN. In one embodiment,nitrogen gas is used to bring MBE chambers to atmospheric pressure forrepairs. Crucibles for materials are baked, as is the entire MBE. In afurther embodiment, a substrate heater of the MBE is raised intemperature during the bake. Up to 1000 C for 10 hours is utilized inone embodiment prior to AlGaN growth.

After treating the MBE machine, a wafer is loaded into a growth chamberof the MBE machine at low pressure, where the wafer is nitridized athigh temperatures and RF exposure. AlN is then deposited at hightemperatures and RF, followed by exposure to Al, Ga and Si with an RFshutter open at temperatures exceeding approximately 1100° C. Thetemperature of the Si is adjusted to obtain different doping densities.The temperature of the Al also controls the mole fraction of the Al,with higher temperatures leading to a higher Al mole fraction.

While the invention was described in terms of doping using an MBEmachine, other processes may also be employed. In one embodiment,undesirable contaminants are also minimized in the other processes.

1. A layer of heavily doped-semiconductor material containing aluminum(Al) and having an electron or hole density greater than approximately1×10²⁰ cm⁻³ wherein the bandgap of the semiconductor material is atleast approximately 4 eV, and wherein the layer has aluminum (Al)compositions at least 50% or higher.
 2. The layer of claim 1 wherein thematerial is high mole fraction AlGaN.
 3. The layer of claim 1, whereinthe material is an AlN layer at least 50 nm thick.
 4. The layer of claim1, wherein the material is an AlN layer is between approximately 50 nmto 150 nm thick.
 5. The layer of claim 4 wherein the layer has aluminum(Al) compositions less than 65%.
 6. A layer of heavily dopedsemiconductor material having an electron or hole density beyonddegeneracy to reduce energy barriers in the wide bandgap semiconductormaterial wherein the bandgap of the semiconductor material is at leastapproximately 4 eV, wherein the layer is doped with impurities, whereinthe impurities comprise at least approximately 1% of volume of thesemiconductor material, and wherein dopant atoms occupy atomic sitesrequired for donor or acceptor behavior.
 7. The layer of claim 6 whereinthe material is selected from the group consisting of AlN, BN, Diamond,carbon nitride, carbon-based nanotubes, diamond-like nitrides,fullerenes, and doped glasses, oxides and ceramics.
 8. The layer ofclaim 7 wherein orbits of valence electrons of donor atoms overlap withone another.
 9. The layer of claim 6 wherein the dopant comprises Si,Be, Mg, Ca, Zn, Sr, or Cd.
 10. A layer of heavily doped widebandgap-semiconductor material containing aluminum (Al) and having anelectron or hole density greater than approximately 1×10²⁰ cm⁻³ whereinthe bandgap of the semiconductor material is at least approximately 4eV, and wherein the aluminum (Al) layer has compositions at least 50% orhigher.
 11. The layer of claim 10, wherein the material is an AlN layerat least 50 nm thick.
 12. The layer of claim 10, wherein the material isan AlN layer is between approximately 50 nm to 150 nm thick.
 13. Thelayer of claim 10 wherein the material is high mole fraction AlGaN. 14.The layer of claim 10 wherein the layer has aluminum (Al) compositionsless than 65%.
 15. The layer of claim 10 wherein doping impurities aremostly located at appropriate donor or acceptor sites in a crystalstructure of wide bandgap semiconductor material.
 16. The layer of claim15 wherein the semiconductor material comprises a very low level ofundesired impurities.
 17. The layer of claim 10 wherein dopingimpurities are mostly located at appropriate donor or acceptor sites ina crystal structure of wide bandgap semiconductor material and whereinthe doping is sufficiently dense to effectively remove ionization energybarriers.